It is known that, in order to carry out their function correctly, spectacles must be positioned in a specific predetermined way relative to the eyes of the person wearing them. This is particularly true of spectacles fitted with corrective lenses (prescription glasses) which must be positioned at specific distances and angles.
One of the parameters which must be met in the positioning of lenses relative to the eyes, and which is particularly critical in the case of progressive lenses, is the pantoscopic angle, that is to say the angle formed by the plane of the lens relative to the vertical plane.
The anatomical characteristics of the person for whom the spectacles are intended may differ widely from one patient to another, making it virtually impossible to construct spectacles that can be adapted in advance to the faces of all possible users.
Moreover, even if the anatomical characteristics of the user are the same, the correct pantoscopic angle also varies according to the type of lenses fitted.
In some cases, the pantoscopic angle is briefly adjusted by the optician at the moment of preparation of the spectacles, by manual adjustment of the arms, which are bent, after being heated if necessary, by making use of the deformability characteristics of their constituent material.
The same procedures are sometimes used for the adjustment of other geometrical and dimensional characteristics of the arms, which must be adapted to the anatomical configuration of the user.
A first of these characteristics is the length of the arm before the curvature of the terminal ends, intended to be supported behind the user's ear.
If this length is excessive, the spectacles tend to slide forwards whenever the head is lowered, whereas if this length is too small, the curved termination bears directly on the ear, causing a painful sensation in the user.
A second dimensional characteristic of the arm that is adapted to the anatomical configuration of the user's head is the inclination of the curvature of the terminal end of the arm towards the head.
Indeed, in many models of spectacles, provision is made to curve the terminal end of the arm slightly towards the other arm of the spectacles, so as to follow the curvature of the head behind the ear and make the positioning of the spectacles more stable.
If the terminal end is excessively inclined, the pressure exerted by it on the head may cause irritation and pain to the user, whereas, if the inclination is small, the gripping action is reduced.
However, this type of adjustment is evidently approximate and usually non-reproducible.
In other types of spectacles, the material or shape of the arms is such that the pantoscopic angle or other dimensional characteristics of the arm cannot be modified, even by using the empirical method described above.
In this case, it may become unavoidable to replace the arm provided on the spectacles with an arm having different dimensions and geometry.
In these circumstances, however, the optician has to provide a suitable selection of different arms for each model of spectacles, thereby increasing the costs and the storage requirements.